The present invention relates generally to capacitive measuring systems and, more specifically, to a system having a sensor shielded against the detrimental effects of external electric fields for capacitively measuring the volume charge density of a sample of material.
Capacitive measuring systems have been used to measure dissolved solids and impurities in fluids such as water and oil. A capacitive sensor is a device having two electrically conductive electrodes and a non-conductive body that insulates the fluid from the electrodes. The electrodes are typically tubular in shape and concentric with one another. When a sample is placed in the body, the device defines a capacitance in response to the dielectric constant of the sample. The dielectric constant varies in response to the ion concentration in the sample, which, in turn, is related to the solid impurities. The capacitance may be measured by connecting a suitable oscillator and measuring circuit to the plates. Comparing the measured capacitance to a known capacitance provides information relating to the electrical properties of the sample. For example, the dissolved solids in a sample of water can be determined by comparing the measured value to that which is produced in response to a known pure (e.g., double-distilled) sample of water.
Conventional capacitive sensors of the type described above are of low precision. They cannot, for example, consistently measure ion concentrations in water below a few parts per million. Practitioners in the art have discovered that measurements may vary over a wide range under seemingly identical test conditions. It would be desirable to provide a capacitive measuring system having a high-precision sensor. This and other problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.